If the engine runs good when cold, but runs poorly when hot, check the EGR valve and the vacuum hose leading to it. The diaphram should not be cracked and it should be free to move the "plunger" in and out. The EGR valve is closed when the car is cold, but is modulated as needed when the engine is up to temperature. If it sticks open, the engine will run poorly at idle or low speed. Carbon build up is the culprit for sticking most of the time, but it can be cleaned off. Also look at the gasket.
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